1. Field of the Invention
The subject matter of this invention is related generally to motor controllers and is related more specifically to microprocessor-controlled motor controllers.
2. Description of the Prior Art
Motor controllers or motor starters as they are sometimes called are well known in the art. Generally, they comprises a switch or ganged switches which are operable to open or close to provide or interrupt current to the stator windings, for example, of an electric motor. The opening and closing process is controlled by appropriately arranged relay coils and relay contacts in what is well known in the prior art as a "relay ladder" arranged in a logical order to properly sequence the starting and/or stopping of the motor.
With the advent of the microprocessor it was found that part, if not all of the relay logic arrangement, could be replaced by a properly programmed microprocessor. Such arrangements are shown and described in a paper entitled "A Quantative Analysis of Grouped Single-Phase Induction Motors" published on page 125 of the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 2, March/April 1981 by J. R. Dunki-Jacobs and Robert H. Kerr; a paper entitled "Thermal Tracking - A Rational Approach to Motor Protection" by D. R. Boothman, E. C. Elgar, R. H. Rehder and R. J. Wooddall identified as IEEE Transactions Paper 274029-5 recommended for presentation at the IEEE PES Winter Meeting, New York, N.Y., Jan. 27-Feb. 1, 1974; a paper entitled "Microprocessor-Based Universal Motor Protection System" appearing in the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 1, Jan./Feb. 1981 by E. B. Turner and H. Michael Willey and in a descriptive bulletin (41-560(E)) entitled MOTOGARD.TM. for motor protection by the Westinghouse Canada Switchgear and Control Division issued Jan. 1980. A reading of all of the above will show that certain factors are of importance when discussing motor starters or motor controllers.
It has always been desirable in the art of motor control to be able to detect certain important information concerning the relationship of the three phases of the power which is applied to the three-phase AC motor. For example, it is very desirable to know if one of the phases has been lost. It is also desirable to know if there is an imbalance in the phases, that is, if one phase is supplying more current or voltage than another phase. Finally, it is important to know if all of the phases have been reversed relative to each other. The presence of any of the aforementioned can cause serious problems in the AC motor being controlled. One of the problems is to detect whether there has been a phase loss or phase imbalance or a phase reversal before the motor is started. Generally, since the contactor which is part of the motor controller is maintained in an opened state before the motor is started, there is no current information available for determining whether there has been a phase loss or a phase imbalance or a phase reversal on the lines supplying the motor. There is however voltage information available. Consequently, any motor controller, whether microprocessor based or not, would have to relay upon the voltage information for determining phase loss, phase imbalance, or phase reversal prior to startup. In the prior art, generally the voltage information is available in terms of both the magnitude of the voltage and the sign of the voltage, that is, whether the voltage is in the positive half cycle or negative half cycle. The magnitude of the voltage generally represents relatively complicated and sensitive information. It is subjected to misinterpretation because of the presence of noise and generally requires a series or set of calculations to provide useful information concerning phase loss, phase imbalance, or phase reversal. It would be desirable if voltage information available at the controller for a motor could be utilized to determine prior to starting the motor whether there was phase loss, phase imbalance, or phase reversal on the incoming power in a manner which was relatively immune to noise, required little or no significant calculation, was reliable, and which further could be utilized to make predictions concerning the percent of phase imbalance if a phase imbalance is present.